Transaminases (E.C. 2.6.1) catalyze the transfer of an amino group, a pair of electrons, and a proton from an amino donor compound to the keto group of an amino acceptor compound. Transaminase reactions can result in the formation of a chiral amine product compound. As shown in Scheme 1, an amino acceptor compound (B) (which is the keto substrate precursor of a desired chiral amine product (D)) is reacted with an amino donor compound (A) in the presence of a transaminase. The transaminase catalyzes the transfer of the primary amine group of the amino donor compound (A) to the keto group of the amino acceptor compound (B). The transaminase reaction results in a chiral amine product compound (D) (assuming R1 is not the same as R2) and a new amino acceptor byproduct (or “carbonyl byproduct”) compound (C) which has a keto group.

Chiral amine compounds are frequently used in the pharmaceutical, agrochemical and chemical industries as intermediates or synthons for the preparation of wide range of commercially desired compounds, such as cephalosporine or pyrrolidine derivatives. Typically these industrial applications of chiral amine compounds involve using only one particular stereomeric form of the molecule, e.g., only the (R) or the (S) enantiomer is physiologically active. Transaminases are highly stereoselective and have many potential industrial uses for the synthesis of optically pure chiral amine compounds.
Examples of the uses of transaminases to make chiral amine compounds include: the enantiomeric enrichment of amino acids (See e.g., Shin et al., 2001, Biosci. Biotechnol. Biochem. 65:1782-1788; Iwasaki et al., 2003, Biotech. Lett. 25:1843-1846; Iwasaki et al., 2004, Appl. Microb. Biotech. 69:499-505, Yun et al., 2004, Appl. Environ. Microbiol. 70:2529-2534; and Hwang et al., 2004, Enzyme Microbiol. Technol. 34:429-426); the preparation of intermediates and precursors of pregabalin (e.g., WO 2008/127646); the enzymatic transamination of cyclopamine analogs (e.g., WO 2011/017551); the stereospecific synthesis and enantiomeric enrichment of β-amino acids (e.g., WO 2005/005633); the enantiomeric enrichment of amines (e.g., U.S. Pat. Nos. 4,950,606; 5,300,437; and 5,169,780); the production of amino acids and derivatives (e.g., U.S. Pat. Nos. 5,316,943; 4,518,692; 4,826,766; 6,197,558; and 4,600,692); and in the production of the pharmaceutical compounds, sitagliptin, rivastigmine, and vernakalant (See e.g., U.S. Pat. No. 8,293,507 B2, issued Oct. 23, 2012; Savile, et al., 2010, “Biocatalytic asymmetric synthesis of chiral amines from ketones applied to sitagliptin manufacture,” Science 329(5989): 305-9; WO2011/159910, published Dec. 22, 2011; and WO2012/024104, published Feb. 23, 2012).
Wild-type transaminases having the ability to catalyze a reaction of Scheme 1 have been isolated from various microorganisms, including, but not limited to, Alcaligenes denitrificans, Bordetella bronchiseptica, Bordetella parapertussis, Brucella melitensis, Burkholderia malle, Burkholderia pseudomallei, Chromobacterium violaceum, Oceanicola granulosus HTCC2516, Oceanobacter sp. RED65, Oceanospirillum sp. MED92, Pseudomonas putida, Ralstonia solanacearum, Rhizobium meliloti, Rhizobium sp. (strain NGR234), Bacillus thuringensis, Klebsiella pneumonia, Vibrio fluvialis (See e.g., Shin et al., 2001, Biosci. Biotechnol, Biochem. 65:1782-1788), and Arthrobacter sp. KNK168 (See e.g., Iwasaki et al., Appl. Microbiol. Biotechnol., 2006, 69: 499-505, U.S. Pat. No. 7,169,592). Several of these wild-type transaminase genes and encoded polypeptides have been sequenced, including e.g., Ralstonia solanacearum (Genbank Acc. No. YP_002257813.1, GI:207739420), Burkholderia pseudomallei 1710b (Genbank Acc. No. ABA47738.1, GI:76578263), Bordetella petrii (Genbank Acc. No. AM902716.1, GI:163258032), Vibrio fluvialis JS17 (Genbank Acc. No. AEA39183.1, GI: 327207066), and Arthrobacter sp. KNK168 (GenBank Acc. No. BAK39753.1, GI:336088341). At least two wild-type transaminases of classes EC 2.6.1.18 and EC 2.6.1-19, have been crystallized and structurally characterized (See e.g., Yonaha et al., 1983, Agric. Biol. Chem. 47 (10):2257-2265).
Transaminases are known that have (R)-selective or (S)-selective stereoselectively. For example, the wild-type transaminase from Arthrobacter sp. KNK168 is considered (R)-selective and produces primarily (R)-amine compounds from certain substrates (See e.g., Iwasaki et al., Appl. Microbiol. Biotechnol., 2006, 69: 499-505, U.S. Pat. No. 7,169,592), whereas the wild-type transaminase from Vibrio fluvialis JS17 is considered (S)-selective and produces primarily (S)-amine compounds from certain substrates (See e.g., Shin et al., “Purification, characterization, and molecular cloning of a novel amine:pyruvate transaminase from Vibrio fluvialis JS17,” Appl. Microbiol. Biotechnol. 61 (5-6), 463-471 (2003)).
Non-naturally occurring transaminases having (R)-selectivity, increased solvent and thermal stability, and other improved properties for the conversion of a wide range of amino acceptor substrates, have been generated by mutagenesis and/or directed evolution of wild-type and other engineered transaminase backbone sequences (See e.g., U.S. Pat. No. 8,293,507 B2, issued Oct. 23, 2012; WO2011/005477A1, published Jan. 13, 2011; WO2012/024104, published Feb. 23, 2012; and Savile, et al., 2010, “Biocatalytic asymmetric synthesis of chiral amines from ketones applied to sitagliptin manufacture,” Science 329(5989): 305-9).
However, transaminases generally have properties that are undesirable for commercial application in the preparation of chiral amine compounds, such as instability to industrially useful process conditions (e.g., solvent, temperature), poor recognition of, and stereoselectivity for, commercially useful amino acceptor and/or amino donor substrates, and low product yields due to unfavorable reaction equilibrium. Thus, there is a need for engineered transaminases that can be used in industrial processes for preparing chiral amines compounds in an optically active form.